DC Isolator for Solar Panels: Do You Need One?

Quick answer

In short

BS 7671 requires a means of isolation on the DC side of a solar inverter, but it doesn’t mention MPPT charge controllers. For a small series string, the IET Code of Practice accepts suitably located plug and socket connectors such as MC4s. We still fit a DC isolator on every system, because the MC4s on a van roof are neither practical nor accessible.

• BS 7671 712.537.2.2.101 and .102: isolate the DC side of the inverter
• IET: a series string can use plug and socket connectors
• An array needs a switch-disconnector or breaker
• Isolator to BS EN 60669-2-4 and/or BS EN 60947-3

On this page 7 sections

1. In short
2. What BS 7671 says about a DC isolator for solar panels
3. IET Code of Practice: Grid Connected Solar Voltaic Systems
4. Table 537.4: Guidance on the selection of protective, isolation and switching devices
5. Do these rules cover campervans and motorhomes?
6. The argument against a DC isolator for solar panels
7. Conclusion: always fit a DC isolator for solar panels

What BS 7671 says about a DC isolator for solar panels

Summarised from BS 7671:2018+A4:2026:

712.537 Isolation and switching

712.537.2 Isolation

712.537.2.101 To allow maintenance and replacement of the inverter, means of isolating the inverter from the DC side and the AC side shall be provided.

712.537.2.2 Devices for isolation

712.537.2.2.101 Isolation on the DC side of the inverter is provided by a switch-disconnector.

712.537.2.2.102 A circuit-breaker suitable for isolation may be used instead.

Our tiny build systems and components differ slightly from the ones BS7671 describes above. That’s because these regulations target PV installations in domestic, commercial and industrial buildings. Those systems use a solar inverter to convert the DC into AC straight away, rather than a solar charge controller and battery.

Don’t confuse a solar inverter with the inverters we commonly use to supply 230V items in our campervans. A solar inverter converts the direct current (DC) a solar panel generates into alternating current (AC) for the grid. These units often have double pole isolators built in, so they don’t need an external DC isolator.

The regulations don’t mention MPPT solar charge controllers (DC/DC) at all. So, as designers, we have no choice but to assume the worst case and include a means of isolation.

Why we fit a DC isolator for solar panels on charge controllers

Suppose the MPPT controller needs replacing or servicing. To call the system ‘safely isolated’, you would have to isolate the DC supply from both the battery and the solar panels. The easiest way to do that is with isolators.

So BS7671 is clear that you do require a DC isolator to isolate the DC supply. However, it only mentions inverters, with no mention of charge controllers or any other electronic devices.

IET Code of Practice: Grid Connected Solar Voltaic Systems

The IET Code of Practice gives us a little more information, and less ambiguity, for solar in campervans, motorhomes and tiny builds. It covers grid connected systems. Campervans and motorhomes have stand alone, off-grid systems, but they often connect to the grid through the shore power inlet. That makes them a grid connected system, so this code of practice is relevant to their installations.

Before we look at the IET Code of Practice table, it’s worth noting these definitions:

PV String: When modules are connected in series fashion, this forms a string. In a string, the voltage adds up though the current remains the same.

PV Array: When modules are connected in series and parallel fashion, this forms an array. In an array, the current adds up when connected in parallel and the voltage adds up when connected in series.

1. PV String: The minimum requirement as a means of isolation for small systems is typically achieved using suitably located plug and socket connectors.

PV strings: is a DC isolator for solar panels required?

Going back to the definitions above, most campervan and motorhome PV installations count as a string. They have a small number of panels, commonly wired in series. So, according to the ‘Notes on application and means of isolation’ section, they technically don’t need an isolator or switch disconnector.

In this case an MC4 connector counts as a ‘suitable plug and socket connector’ and will be enough. Just remember that this is a minimum requirement. For both safety and practicality, we still recommend DC isolators on all our systems.

The main reason we don’t use the MC4 connector as the means of isolation is location. The connectors must sit somewhere practical and accessible. On most campervans and motorhomes they are on the roof, which is neither practical nor readily accessible.

2. Array: Isolation is typically achieved by a switch-disconnecter or circuit breaker, either of which may be built into the inverter or mounted adjacent to it. (if necessary in a suitable enclosure).

PV arrays need a DC isolator for solar panels

Again, going by the definitions above, a parallel connected set of modules (solar panels) would arguably count as an array. In that case it needs isolation on the DC side. The IET note above says:

‘Isolation is typically achieved by a switch-disconnecter or circuit breaker.’

Table 537.4: Guidance on the selection of protective, isolation and switching devices

We always recommend an isolator, whatever your solar panel setup. The isolator for your PV panels must meet the British Standards in the table above.

To comply with the British Wiring Regulations, the isolator must meet BS EN 60669-2-4 and/or BS EN 60947-3.

Do these rules cover campervans and motorhomes?

The best practice and regulations above don’t specifically consider campervans and motorhomes. That doesn’t stop us using them to stay compliant. In fact, we use them to make sure our installations go beyond the minimum requirements.

BS7671 Section 721, Electrical installations in caravans and motor caravans focus directly on the electrical systems within motor caravans. This section merely notes the deviations from the rest of BS7671, so we’re seemingly left to read between the lines here.

A solar panel or charge controller in a van is more likely to get damaged or need maintenance than one in a house. After all, buildings don’t move, vibrate or get driven down a bumpy track to the best surf spot! That is one of the key reasons to install an isolator.

The argument against a DC isolator for solar panels

One argument against DC isolators is that they add resistance to the solar panel cables. That reduces the cable’s current carrying capacity, so the MPPT solar charge controller can’t produce its optimum charge current. There is some truth in this. However, the way the cables are terminated into the isolator plays a big part in keeping the resistance down.

In most installations we see, someone has twisted the multi stranded solar cable together and poorly terminated it into the DC isolator’s contacts. The strands are damaged or missing, because stripping the insulation put pressure on the cores.

Ferrules are essential here. A ferrule holds all the strands together, so it makes a better contact inside the isolator. It also makes it easier for the installer to locate the cable in the terminal.

Conclusion: always fit a DC isolator for solar panels

We take a ‘belt and braces’ approach and always fit a DC isolator for solar panels, between the panels and the solar charge controller.

If the solar charge controller needs isolating for maintenance or replacement, one flick of the isolator safely isolates the MPPT from the panels. Disconnecting MC4 connectors, on the roof or in a cupboard, is more work and less safe.

DC isolator for solar panels FAQs